Clean the liver, cut into cubes and fry a little in a part of butter. Cut thinly onion and mushrooms, fry in butter, add tomato puree and cook through for 7 minutes.

Cook spaghetti in a large amount of salted water for 3 minutes, cast in a colander. Put liver in portion pots, add spaghetti, onion with mushrooms, salt, pepper, pour with stock and season with greens. Cover with lid and stew in oven for 20 minutes.